Magnetic random access memory (MRAM) that incorporates an MTJ as a memory storage device is a strong candidate to provide a high density, fast (1–30 ns read/write speed), and non-volatile solution for future memory applications. An MRAM array is generally comprised of an array of parallel first conductive lines on a horizontal plane, an array of parallel second conductive lines on a second horizontal plane spaced above and formed in a direction perpendicular to the first conductive lines, and an MTJ interposed between a first conductive line and a second conductive line at each crossover point. A first conductive line may be a word line while a second conductive line is a bit line or vice versa. Alternatively, the first conductive line may be a sectioned line which is a bottom electrode. There are typically other devices including transistors and diodes below the array of first conductive lines.
The MTJ consists of a stack of layers with a configuration in which two ferromagnetic layers are separated by a thin nonmagnetic insulating layer such as Al2O3 or AlNxOy which is called a tunnel barrier layer. One of the ferromagnetic layers is a pinned layer in which the magnetization (magnetic moment) direction is more or less uniform along a preset direction and is fixed by exchange coupling with an adjacent anti-ferromagnetic (AFM) pinning layer. The second ferromagnetic layer is a free layer in which the magnetization direction can be changed by external magnetic fields. The magnetization direction of the free layer may change in response to external magnetic fields which can be generated by passing currents through the conductive lines. When the magnetization direction of the free layer is parallel to that of the pinned layer, there is a lower resistance for tunneling current across the insulating layer (tunnel barrier) than when the magnetization directions of the free and pinned layers are anti-parallel. The MTJ stores information as a result of having one of two different magnetic states.
In a read operation, the information is read by sensing the magnetic state (resistance level) of the MTJ through a sensing current flowing through the MTJ, typically in a current perpendicular to plane (CPP) configuration. During a write operation, the information is written to the MTJ by changing the magnetic state to an appropriate one by generating external magnetic fields as a result of applying bit line and word line currents. Cells which are selectively written to are subject to magnetic fields from both a bit line and word line while adjacent cells (half-selected cells) are only exposed to a bit line or a word line field. Due to variations in MTJ size and shape that affect the switching field of a free layer, a magnetic state in a half-selected cell may be undesirably altered when writing to a selected cell.
To preserve data (magnetic state) against erasure, an in-plane magnetic anisotropy has to be strong enough in the storing magnetic layer. Current designs are based on shape anisotropy involving rectangular, ellipse, eye, and diamond-like patterns. A problem with these designs is that coercivity is highly dependent on shape, aspect ratio, and MTJ cell size and is therefore very sensitive to cell shape and edge shape which are subject to variations because of cell patterning processes. As a result, MTJ cell differences make the switching field highly variable and difficult to control.
To compete against current DRAM, SRAM, and Flash technologies, MRAM cell size must be in sub-micron dimensions. However, for sub-micron cell sizes, thermal agitation can switch the cell magnetization randomly, especially for half-selected cells. To prevent thermal agitation of half-selected cells, greater magnetic anisotropy is required which in turn demands a very high write current that renders MRAM non-competitive against other existing technologies because of high current and power consumption. Therefore, an alternate means of providing higher magnetic anisotropy is needed to make MRAM useful for high density and high speed applications.
Referring to FIG. 1, a conventional MRAM array I comprised of two adjacent MRAM cells with two MTJs 4 is depicted. There is a substrate 2 with a first conductive layer that in this example includes bottom electrodes 3 formed therein. Each bottom electrode 3 contacts an overlying MTJ 4 which is enclosed on the sides by an insulation layer 5. In this example, there is a bit line 6 in a second conductive layer that contacts the top of the MTJs 4. Typically, a second insulation layer 7 is deposited on the second conductive layer including bit line 6 and is subsequently planarized with a chemical mechanical polish (CMP) process. A third conductive layer 9 which may be an array of word lines is formed within a third insulation layer 8 on the second insulation layer 7. There are other circuits (not shown) that are used to select certain MTJs for read or write operations.
Referring to FIG. 2, a typical MTJ 4 is shown which is a stack of layers including one or more bottom seed layers 10 such as NiFeCr formed on a bottom electrode 3. Next, an anti-ferromagnetic (AFM) pinning layer 11 that may be PtMn, for example, is deposited on the seed layer 10. There is a ferromagnetic “pinned” layer 12 also known as a reference layer on the AFM layer 11 that may be a composite of multiple layers including CoFe layers. The tunnel barrier layer 13 above the pinned layer 12 is generally comprised of an insulating material such as Al2O3. Above the tunnel barrier layer 13 is a ferromagnetic “free” layer 14 which may be another composite layer that includes NiFe, for example. At the top of the MTJ stack is one or more cap layers 15. In configurations where only one cap layer is employed, the cap layer 15 is comprised of conductive material such as Ru for making an electrical contact to the subsequently formed bit line 6. This MTJ stack has a so-called bottom reference layer configuration. Alternatively, the MTJ stack may have a top reference layer configuration in which a free layer is formed on a seed layer followed by sequentially forming a tunnel barrier, a reference layer, an AFM layer, and a cap layer.
In U.S. Pat. No. 6,654,278, a process is used to form a magnetization vortex with a net magnetic moment of zero in a reference magnetic region of an MTJ. An applied magnetic field causes the vortex center to shift in a direction orthogonal to the bit easy axis or to the net applied field.
An MTJ device is disclosed in U.S. Pat. No. 6,269,018 in which a free layer and pinned layer are approximately circular (isotropic) in shape and have a magnetization in the form of a vortex. When a write current flows through the MTJ, a self-field is produced that changes the magnetization vortex state of the free layer from a first predetermined handedness to a second predetermined handedness.
A magnetic element is described in U.S. Patent Application Publication 2004/0021539 in which a closed loop of ferromagnetic material has an even number of magnetic domains of opposite sense. For data storage, two circular loops with notches are stacked with one on top of the other wherein the domains of one loop are either parallel or anti-parallel to the domains of the second loop.
In U.S. Patent Application Publication 200210196658, a spin vortex is formed in a circular shaped first magnetic film of a storage element and then a vertical magnetization is generated at the center of the spin vortex. A second magnetic film has a magnetization perpendicular to its top and bottom surfaces.
A method of toroid reading and writing is disclosed in U.S. Pat. No. 6,266,289 in which a toroid element is interposed between two or four current conducting biasing busbars. One of the busbars extends into the axial opening of the toroid element to generate a vortex magnetic field therein and a second busbar generates a magnetic field that is transverse relative to the vortex field.